Electrochemical capacitors (ECs) often called as “Supercapacitors” have been considered to be one of the most important power sources in several devices including memory back-ups, hybrid power systems for electric vehicles, military and medical applications, digital communications and are currently widely investigated because of their interesting characteristics in terms of high power densities and long cycle life.
ECs are electrical devices with highly reversible charge storage and delivery capabilities. ECs have properties complementary to secondary batteries and are composed, e.g., of carbon based electrodes and an electrolyte.
An electrolyte is any substance containing free ions that behaves as an electrically conductive medium. Because electrolytes generally consist of ions in solution, electrolytes are also known as ionic solutions, but molten electrolytes and solid electrolytes are also possible. ECs and other energy charge storage devices, such as batteries, employ both aqueous and non-aqueous electrolytes in either liquid or solid state. The performance of the supercapacitor is related to the characteristics of the electrode material and the electrolyte employed in the device. The energy density (Wh/kg) of the supercapacitor is expressed as
      Energy    ⁢                  ⁢    density    ⁢                  ⁢          (              Wh        /        kg            )        =            1      8        ⁢          (              F        /        g            )        ×                  V        0        2            3.6      
Where F/g is the specific capacitance of the electrode material and V0 is the cell voltage dependant primarily on the electrolyte used in the device. Liquid electrolytes like aqueous electrolyte solution and organic electrolyte solution are the commonly used as electrolyte in EC. Among these, aqueous electrolytes are popular, because of low cost, ease of fabrication in ambient conditions. However decomposition voltage of aqueous solvent limits the supercapacitor cell voltage to 1.0 V. Non-aqueous or organic electrolytes based supercapacitors provide higher operating voltage (2 to 3 V) thus possess higher energy density but are expensive and require handling under controlled atmospheres (glove box) to keep it pure/dry. One of the main concerns of both organic and aqueous liquid electrolyte based supercapacitors is the risk of electrolyte leakage affecting the device reliability and safety. On the other hand solid electrolytes consisting of polymer or polymeric gel electrolyte circumvent this issue and provide advantages of compactness and reliability without leakage of liquid components. However, ionic conductivity of these electrolytes (in the range of 10−10 to about 10−7 S*cm−1) is too low to be of any use in practical devices. Hence polymer electrolytes under investigation contain salt additives which are entrapped in the polymeric gel moiety to improve its conductivity.
Although, present day printed electronics devices can be made using simple roll-to-roll printing processes, conventional batteries/supercapacitors are not fully compatible with devices such as smart cards, electronic paper, wearable electronics, and the like. Ideally these necessitate printable supercapacitors and batteries that can be directly printed on to surface of electronics and thus can be directly integrated in to disposable displays like RFID tags. Considerable effort worldwide has been directed on the research and development of suitable electrode materials for printable flexible supercapacitors. However, these first generation prototypes still consist of liquid based electrolyte or polymer electrolyte consisting of a liquid component. This is a major drawback as such liquid based components will require additional encapsulation that will hinder the flexibility and/or printability of the charge storage device in sync with the printed electronics.
Hence there is an ongoing demand to obtain suitable candidate materials that can act as electrolyte without compromising on the device performance.